US president Barack Obama may have returned to Washington to prepare for his second term but his electoral race with Republican challenger Mitt Romney in Florida remains too close to call.

More than 24 hours after polls closed in Florida, election officials said votes were still being counted in a handful of counties and final results may not be known before the weekend.

"Every county must report their unofficial results to us by Saturday at noon," Chris Cate, a spokesman for Florida's secretary of state, who is responsible for elections, said.

He declined to predict when the race in the fourth most populous US state would be called.

Twelve years ago, when the key battleground state was a toss-up that left the presidential race unsettled, Florida was the cause of electoral gridlock.

This time, it hardly seemed to matter after Mr Obama handily won re-election without Florida's 29 electoral college votes, which was the biggest prize up for grabs in any of the US swing states.

As of Wednesday evening (local time), Mr Obama had 49.87 per cent of the state-wide vote compared to 49.27 per cent for Mr Romney, with just 49,963 votes separating them, according to the Florida Division of Elections.

Republican governor Rick Scott's decision not to extend early voting ahead of election day, after it was cut back from 14 to eight days was cited as causing exceedingly long voter lines at many precincts on Tuesday.

Democrats have said repeatedly that the cutback was a part of an unsuccessful attempt to blunt turnout in Florida by Mr Obama's supporters.

Miami-Dade mayor Carlos Gimenez apologised for the long lines in his county on Wednesday, after acknowledging that some voters had been forced to wait up to six hours to cast their ballots.

"That should not have happened," Mr Gimenez, whose county accounts for about 10 per cent of Florida's nearly 12 million registered voters, said.

"We had a very long ballot. It was the longest ballot in Florida history."

The final margin of victory in Florida may be less than 1 per cent.

Some political pundits say the delays highlight Florida's seeming inability to hold elections that are free of controversy and public mockery.

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